1. Field of the Invention
The present invention relates to cascaded optical amplifiers and, more particularly, to gain and signal level adjustments of cascaded optical amplifiers.
2. Description of the Related Art
Optical communication systems using optical fiber transmission lines are being used to transmit relatively large amounts of information. For this purpose, low-loss (e.g., 0.2 dB/km) optical fibers have been manufactured and used as optical fiber transmission lines. In addition, optical amplifiers are used to compensate for losses in the optical fiber transmission line to thereby allow long-haul transmission.
A conventional optical amplifier includes an optical amplifying medium which is pumped with pump light to provide a gain band. The optical amplifying medium and pump light are chosen so that they provide a gain band which includes a wavelength of signal light. As a result, the signal light will be amplified as the signal light travels through the optical amplifying medium. For example, an erbium doped fiber amplifier (EDFA) includes an erbium doped fiber (EDF) as the optical amplifying medium. A pumping light source supplies pump light having a predetermined wavelength to the EDF. By setting the wavelength of the pump light within a 0.98 μm band or a 1.48 μm band, a gain band including a wavelength band of 1.55 μm can be obtained. Therefore, signal light in the 1.55 μm wavelength band will be amplified.
Another type of conventional optical amplifier has a semiconductor chip as the optical amplifying medium. In this case, pumping is performed by injecting an electric current into the semiconductor chip.
Further, wavelength division multiplexing (WDM) is a known technique for increasing transmission capacity through a single optical fiber. In a system adopting WDM, a plurality of optical carriers having different wavelengths are individually modulated with data. Thus, each modulated carrier represents a channel of the WDM system transmitting an optical signal. The optical signals (that is, the modulated carriers) are then wavelength division multiplexed by an optical multiplexer to obtain WDM signal light. The WDM signal light is then transmitted through an optical fiber transmission line. The WDM signal light is received through the transmission line, and then demultiplexed into individual optical signals by an optical demultiplexer. Data can then be detected from these individual optical signals. Therefore, by applying WDM, the transmission capacity of a single optical fiber can be increased in accordance with the number of WDM channels multiplexed together and transmitted through the optical fiber.
When an optical amplifier is inserted along the transmission line in an optical communication system adopting WDM, a transmission distance is limited by the wavelength characteristic of gain which is represented by a gain tilt or gain deviation of the optical amplifier. For example, in an EDFA, it is known that a gain tilt is produced at wavelengths in the vicinity of 1.55 μm, and this gain tilt varies with total input power of signal light and pump light power to the EDFA.
There is an optical amplification device for optical amplification which can maintain the wavelength characteristic of gain constant and obtain a wide input dynamic range. The optical amplification device includes first and second optical amplifiers and a variable optical attenuator optically connected between the first and second optical amplifiers. Automatic gain control (AGC) is applied to each of the first and second optical amplifiers, thereby maintaining constant the wavelength characteristic of gain of each of the first and second optical amplifiers. Further, automatic output level control (ALC) is performed for the second optical amplifier by using the variable optical attenuator, thereby obtaining a wide input dynamic range. That is, the output level of the second optical amplifier is maintained constant irrespective of the input level of the first optical amplifier, so that the input dynamic range of this device is widened.
In such an optical amplification device, AGC is performed so that the gain of the first optical amplifier is maintained constant irrespective of the input level of the first optical amplifier. Accordingly, there arises a problem such that when the power of signal light to be supplied to the first optical amplifier is increased, the power of pump light must be increased by the corresponding amount to increase the output power of the first optical amplifier to provide the required gain. That is, a high-power pumping light source is required for the first optical amplifier to ensure a required input dynamic range.